The only disadvantage is that you have to use Alt instead of
AltGr to get the \ Backslash (on a German
Keyboard)

You have to call your xpcterm with the option
-name XEga -fn ega

Switching between X11R5 and X11R6 configuration

to compile X11-Clients as either R6 or R5 clients,
should be as easy as you only switch the PATH components
so that either /usr/X11R6/bin/xmkmf or
/usr/X386/bin/xmkmf would make the new Makefile.

ISC Streams Pipes

The old path to the pipes on ISC's R4 /tmp/.X11-unix
has changed to /dev/X/ISCCONN. For compatibility
reasons on ISC, the pipes in the new directory will be
linked to a file inside the old.
This will normally be a hard link, so it can't go across
filesystems. On ISC Version 4.x this is now allowed. But
you should use the new S5L on both filesystems.
ISC30 systems should take care that the two directories
are on the same FS. Else if you are using a ISC40 compiled
binary, the Server could maybe abort due to a SIGSYS.
We tried to catch this signal, so if it dumps please send
me a note.

Warnings you may see:

Since 3.2A, you could see a warning from pre X11R6.3 clients.

Warning: Unable to load any usable fontset

The case are the new gzipped fonts, but the Warning isn't serious.

If you start a server you may see the following message:

_XSERVTransOpen: transport open failed for named/enigma:0
_XSERVTransMakeAllCOTSServerListeners: failed to open listener for named

This message either isn't critical. Interactive doesn't support this
kind of connection.